This invention relates to the synthesis of structured carbons. More particularly, this invention relates to a method of synthesis of mesoporous and other carbons by imprinting precarbon particles which are then carbonized.
There is great interest in the synthesis of mesoporous carbons because of their applications in adsorption and separation of large organic compounds, double layer capacitors and rechargeable batteries. Mesoporous carbons are generally considered to be carbons with pores having diameters between about 2 nanometers (nm) and about 50 nm. Although porous carbons have been in use for thousands of years and have become industrially important in various areas, methods to design their porous structures are relatively recent. One such method employs ordered mesoporous silicas (OMS), e.g., MCM-48 and SBA-15, as templates. According to these methods, the pores of a silica are filled with a carbon precursor such as sucrose, poly(furfuryl alcohol), or phenolic resin, followed by carbonization of the carbon precursor and dissolution of the silica. The resulting carbons exhibited ordered mesopores of a size below about 7 nm, arising from the dissolution of the silica framework. However, carbons synthesized from OMS of different pore sizes exhibit similar pore width because the pore size in these carbons is determined by the wall thickness of the silica templates, which is usually about 1-3 nanometers (nm).
Other methods, which are more appropriate for the preparation of macroporous materials, involve the use of silica colloids as templates. Colloidal silica particles and colloidal crystals have both been used for the preparation of porous carbons. Colloidal silica particles may be used to synthesize mesoporous carbons by mixing colloidal silica particles with a fluid-type carbon precursor to obtain a composite aggregate, subsequent carbonization of the composite, and silica dissolution. This method provides a carbon with disordered pores resembling the initial colloidal aggregates.
In contrast, the colloidal crystal templating method provides carbons with ordered macropores (pores greater than about 50 nm) or mesopores of size greater than 30 nm. This templating route involves the formation of colloidal crystals, infiltration of the crystal's interstitial space with a fluid-type carbon precursor and its solidification followed by removal of the template. A common feature of the aforementioned templated carbons is the presence of disordered micropores (pores with diameters less than 2 nm) in the walls of ordered macropores or mesopores. These micropores are formed during carbonization of many common carbon precursors.
Therefore, a need exists for a method of synthesizing mesoporous carbons having uniform mesopores with negligible microporosity. A need also exists for a method of synthesizing mesoporous carbon that permits the pore size to be adjusted over a wide range of mesopores, i.e., from about 5 nm to 30 nm or more, and which allows the pore volume and surface area to be tailored.